A water-absorbing resin (SAP/Super Absorbent Polymer), which is a water-swellable and water-insoluble polymeric gelling agent, exhibits an excellent characteristic for absorbing body fluids. For this reason, a water-absorbing agent containing a water-absorbing resin as a main component is often used for absorbent articles such as a disposable diaper or a sanitary napkin, an agricultural water-retaining agent, an industrial water sealant, or the like. Many kinds of monomers and hydrophilic polymers have been proposed as a raw material of such a water-absorbing resin contained in an absorbent agent. However, a poly (meth)acrylic acid (salt)-based water-absorbing resin using (meth)acrylic acid and/or a salt thereof as a main component is often used in industry from the viewpoint of price and performance.
Such a water-absorbing resin is produced through a polymerization step, a drying step, an optional step of removing a non-dried product, a crushing step, a classification step, a surface crosslinking step and the like (Non-Patent Literature 1).
For disposable diapers, which are a primary application for a water-absorbing agent, remedies for urine leakage and skin rash are in need. As a method for examining these problems, a method for measuring a re-wet amount from a disposable diaper under pressure and a method for measuring liquid absorption time of a disposable diaper under pressure have been proposed.
It is estimated that, when urine is hardly incorporated into a disposable diaper while the body weight of a wearer of a disposable diaper is applied, or when a water-absorbing agent absorbs urine slowly even if urine is incorporated into the disposable diaper, urine leakage or skin rash occurs. Then, it is considered that improvement in absorbability of a water-absorbing agent under pressure and improvement in water absorbent speed of the water-absorbing agent lead to reduction in the re-wet amount of a disposable diaper and the liquid absorption time, and further lead to reduction in urine leakage and skin rash.
Conventionally, in order to reduce the re-wet amount of a disposable diaper and to reduce the liquid absorption time of the disposable diaper, many techniques for improving absorption characteristics under pressure have been proposed.
Specific examples of the proposition include a technique for using a water-absorbing agent having a large sum of absorption capacities under four different kinds of loads (PAI) for a disposable diaper (Patent Literature 1), a technique for improving diffusivity of a liquid not only in a vertical direction but also a horizontal direction in a SAP layer under pressure (Patent Literatures 2 and 3), a technique for improving fluid retention capacity under pressure while the SAP amount per unit area is large (Patent Literatures 4 and 5), and a technique for improving fluid retention capacity under pressure measured while there is a difference in height between a glass filter in contact with a water-absorbing agent and a liquid surface on the liquid supply side (Patent Literature 6).
Each technique of these Patent Literatures 1 to 6 has a long measurement time, and evaluates the fluid retention capacity under pressure in a saturation state. Improvement of these problems makes it possible to reduce the re-wet amount of a disposable diaper to some degree and to shorten the liquid absorption speed. However, the effect is still insufficient, and a new evaluation parameter has been demanded.
In order to improve the time required for a water-absorbing agent to absorb urine (absorbing time), many techniques to improve the particle shape have been proposed so far.
Specifically, a technique for improving a water absorbent speed by improving a specific surface area is known. Examples thereof include a technique for controlling the particle diameter finely (Patent Literature 7), a technique for performing surface crosslinking after controlling a particle diameter finely and finally performing granulation with a small amount of water (Patent Literature 8), a technique for crushing a polymerization gel with a specific energy to form particles having a granulated form (Patent Literatures 9 and 10), a technique for granulating water-absorbing resin fine particles having a large surface area using a binder (Patent Literatures 11 to 16), a technique for mixing water-absorbing resin fine particles having a large surface area with water or hot water at a high speed for granulation (Patent Literatures 17 and 18), a technique for performing polymerization in multiple steps to obtain granulated spherical particles when producing a water-absorbing resin by a reverse phase suspension polymerization method (Patent Literatures 19 and 20), a technique for lyophilizing a hydrogel to make the hydrogel porous (Patent Literature 21), a technique for performing foaming polymerization using a carbonate (Patent Literatures 22 to 29), a technique for foaming using an organic solvent (Patent Literatures 30 and 31), a technique for foaming by introducing an inert gas into an aqueous monomer solution (Patent Literatures 32 to 36), a technique of polymerization while a gas generated by raising the temperature of an aqueous monomer solution in the presence of a surfactant is dispersed in the aqueous monomer solution (Patent Literatures 37 and 38), a technique for using an azo compound (Patent Literatures 39, 40, and 41), a technique for using insoluble inorganic powder or water-insoluble particles (Patent Literatures 42 and 43), and a technique for polymerizing a slurry without stirring, in which a fine precipitate of sodium (meth)acrylate salt containing microbubbles of an inert gas and having a concentration of 45 to 60% by weight is dispersed (Patent Literature 44). Further, examples thereof include a technique for foaming and crosslinking after polymerization (Patent Literature 45) and a technique for foaming by adjusting the wind velocity or the like during drying (Patent Literature 46).
When the particle diameter is simply reduced as in the Patent Literature 7, an effect for improving the water absorbent speed can be observed. However, deterioration of handleability due to the increase of fine powder, particularly deterioration of handleability under moisture absorption, is observed significantly. In recent years, with spread of disposable diapers, it has become essential to produce disposable diapers in a humid area, and such deterioration of handleability has been considered to be unacceptable. Further, in a water-absorbing agent having a small particle diameter, the following problems have been observed. That is, when an absorbent material is formed by laminating pulp and a water-absorbing agent, the amount of the water-absorbing agent falling off from the absorbent material is increased, the working environment is deteriorated, and the liquid absorption time of a disposable diaper becomes longer.
When the water absorbent speed is improved by a granulation method as in the techniques of the Patent Literatures 8 to 18, an effect for improving the water absorbent speed can be observed to some extent. However, many techniques are not highly effective, that is, the granulated particles have low strength, and the granulated particles easily return to the original fine powder in a dry or swollen state. Further, it has been similarly observed that handleability is deteriorated particularly under moisture absorption by the increase of the surface area of a water-absorbing agent due to granulation.
Moreover, as in the techniques of the Patent Literatures 19 and 20, when granulated particles are formed by a reverse phase suspension polymerization method, an effect for improving the performance of a disposable diaper is insufficient. For example, there are problems of a complicated process and a residual organic solvent, as well as the problem that the re-wet amount is increased due to reduction in a surface tension.
In the methods for improving the water absorbent speed by a foaming method described in the Patent Literatures 21 to 46, an improvement effect can be observed to some degree, but many techniques do not exhibit a sufficient effect at all, and make cost higher due to the use an expensive raw material or a special apparatus. Further, there is a problem that a surfactant used to disperse bubbles reduces the surface tension and increases the re-wet amount. Moreover, since a water-absorbing agent subjected to foaming increases the surface area and fine powder similarly to the case of granulation, it has been similarly observed that handleability is deteriorated particularly under moisture absorption.
According to the related art, as a technique for imparting fluidity under moisture absorption (technique for suppressing blocking after moisture absorption), many techniques have been already proposed. Specific examples thereof include a technique for adding water-insoluble inorganic fine particles (Patent Literatures 47 and 52), a technique for adding a polyvalent metal salt of an organic acid having seven or more carbon atoms in a molecule thereof (Patent Literature 48), a technique for associating a water-insoluble metal phosphate on the surface (Patent Literature 49), a technique for treating with a specific silicone surfactant (Patent Literature 50), and a technique for mixing clay into a water-absorbing resin (Patent Literature 51).
However, it has been known that, only by applying the techniques of the Patent Literatures 47 to 52 for imparting fluidity under moisture absorption to the techniques of the Patent Literatures 1 to 46, the re-wet amount is increased and a problem such as occurrence of urine leakage or skin rash increases, while handleability can be improved.